In most steering systems which currently equip motor vehicles a steering pinion is linked in rotation with a steering column which is operated by means of a steering wheel of the vehicle, often with hydraulic or electric powering. The steering pinion meshes with a rack mounted so that it slides in longitudinal direction inside a steering housing. The two ends of the rack, outside the housing, are respectively coupled with two tie rods which themselves are respectively associated with left and right steerable wheels of the vehicle. Therefore the rotation of the steering wheel in one or other direction transmitted by the steering column to the pinion is converted into corresponding translation of the rack which, via the tie rods, causes directing of the steerable wheels to turn to the right or to the left.
In said steering system, the rack-and-pinion mechanism, linked to the front drive train of the vehicle, via the tie rods, is subjected to upward forces, impacts and vibrations depending on the state of the roadway on which the vehicle is travelling. On account of the angle formed by the tie rods with the rack, forces may be exerted on the rack which may move it away from the pinion. For this reason, the rack is usually applied permanently against the pinion by means of a device called a pusher which acts elastically on the back of the rack in the region of the pinion to press the teeth of this rack strongly against the pinion. The pusher therefore limits any play between the respective teeth of the pinion and rack, and this pusher also provides control over the sliding force of the rack inside the housing. The action of the pusher also allows compensation for any straightness defects of the rack.
In its most frequent embodiment, the pusher device comprises the actual pusher which is a rigid part mounted mobile in translation in a direction substantially perpendicular to the longitudinal axis of the rack, and urged towards the back of the rack by spring means which are also arranged in a direction substantially perpendicular to the longitudinal axis of the rack.
On the contrary, with a so-called off-centred pusher device such as described in patent U.S. Pat. No. 6,247,375 B1 (or in patent documents FR 2219868 and EP 0770538 A2), a rotatable yoke comprises an eccentric part which pushes the rack towards the pinion, the rotatable yoke being mounted in rotation in a housing such as a pinion housing around an axis parallel to the longitudinal axis of the rack. The inner periphery of this yoke is eccentric relative to its outer periphery so that when it rotates in the housing its eccentric part is applied against the back of the rack and presses it towards the teeth of the pinions so that they remain meshed. The yoke is urged in rotation in one direction by a spring (see in particular the above-cited document FR 2 219 868) so as to make up for any play caused by assembly imprecision and by wear between the teeth of the rack of the rack and the teeth of the pinion.
With regard to above-mentioned patent U.S. Pat. No. 6,247,375 B1, the inner part of the rotatable yoke comprises a recess in which a plate spring is mounted which bears against the rack and therefore allows slight radial movement to offset straightness defects of the rack.
The advantages of an eccentric pusher device particularly lie in the compactness of said device and in the fact that it can be made irreversible, any backward return of the rotatable support (when a certain angle position is reached) being made impossible.
However, in the arrangement according to patent U.S. Pat. No. 6247375 B1, the fact that the plate spring is placed between the rotatable yoke and the rack means that when the rack is subjected to variable forces it does not always bear in similar manner upon the spring, which may cause losses of contact between the said spring and the rack. In addition, the plate spring changes position over time having regard to the progressive rotation of the yoke, which causes variation in the orientation and intensity of the forces applied to the rack. This may all lead to strong degradation of the function performed by the off-centred pusher device.